1. Field of the Invention
The present invention relates to a method and system for producing liquid water from moisture in air.
2. Description of the Related Art
Fresh water is a finite resource which the global population is exhausting at a dangerously unprecedented rate. Developed nations have long been able to look at water problems as only being relevant to underdeveloped nations, desert areas and agrarian societies, but this is quickly becoming an outdated view. In recent years, overwhelming emphasis has been placed on energy issues, but regrettably little emphasis has been placed on fresh water. Currently over a billion people lack access to any clean water and this is part of a tend that is only increasing as the global population rises. From 1900 to 1995, the global population more than tripled, while demand for fresh water has disproportionally increased over six fold. Unfortunately, this unbalanced supply and demand shows no signs faltering. It is actually quite the opposite. Within the next 35 years the world population will increase by 40 to 50% and by 2030. The Water Resources Group estimates that global water demand will outstrip supply by 60% unless a solution is found.
Given the grave state of water availability, a need exists for a timely and economical solution. More particularly, as more people move inland and as cities become too dense, there is a need for an alternative avenue for generating clean water, and more particularly for a portable water supply for those areas where water is not readily found. The UN estimates that on average every US dollar invested in water and sanitation provides an economic return of eight US dollars. This is only the current rate, which merely takes into account the current demand and inefficient means of providing water. It is not only the poor rural regions of the world who are incapable of meeting their demand for water. The global urbanization rate has 60 million people moving into cities each year and in developing countries like China and India the rate is even higher. From Singapore to San Antonio, urban populations are rising and water is an increasingly scarce and precious commodity. While the water problems of the past were largely out of sight and out of mind, dispersed amongst low population rural areas, they are increasingly at the front door step of many nation's population centers. As the economies of these nations grow, there is an increasingly large class of people in them with the means and hard felt need for a technology to provide a clean water stream.
In 2012, for the first time ever, farmers in northwest Texas have had quotas placed on how much water they can pump, a situation all too common in many developing countries. This has and will continue to create strains between industries (e.g. oil & gas vs. agricultural vs. power producers) and between states (Georgia, Alabama, and Florida are in a dispute over water for drinking, recreational, farming, environmental purposes, and hydropower in the Apalachicola-Chattahoochee-Flint River system). The stress created by these conflicts will stifle cooperation when there is an increasing need of it to solve the water issue. Thirty percent of U.S. irrigated farmland depends on the Ogallala Aquifer, which is rapidly being depleted. In some areas, the water table is dropping by as much as two feet per year. In 2005, the USGS estimated that total water volume was about 2,925,000,000 acre feet (3,608 km 3). This is a decline of about 9% since significant ground-water irrigation development began. Some geologists fear the aquifer could dry up in as little as 25 to 30 years. Once the Ogallala is drained, it will take over 6,000 years to recharge with rainfall.
This decreasing water supply is not just pressing for cities or large scale farming. Sixteen percent of US citizens live outside of cities and rely on personal wells for water. Texas alone has 3.6 million rural residents. Typically, these people are forced to spend valuable time and money digging wells and softening their water or face having have poor tasting, poor quality water that will drastically shorten the lifespan of their air conditioners and washing machines resulting in hundreds of dollars more in repair and replacement costs. These wells source their water from the ground and as sea levels are rising, they are polluting these fresh water sources with salts and other contaminants. Those provided with water from surface sources are also facing increased threats. As heavy precipitations become more frequent & violent, larger volumes of sediments become suspended in water, reducing its quality. Furthermore, higher air temperatures are leading to higher water temperatures which lead to longer period of summer stratification (when surface and bottom water do not mix). This can cause lower levels of oxygen in the water which among others issues, decreases the purification capabilities of rivers. As such, the stress of fresh water will increase on several fronts.
As surface and well water supplies are hastily being depleted, many governments and municipalities are turning their attention towards desalination. According to the International Desalination Association, in 2009, 14,451 desalination plants operated worldwide, producing 59.9 million cubic meters (2.12×109 cubic feet) per day, a year-on-year increase of 12.3%. It was 68 million cubic meters in 2010 and expected to hit 120 million cubic meters by 2020; 40 million cubic meters of which is planned for the Middle East alone. Unless an improved technology is implemented, this trend will only increase. Interestingly, over 60% of the cost in desalination is a result of the upstream cost, the treatment and filtration of the seawater in preparation for reverse osmosis. However, this is only a fraction of the problem with existing desalination technology, as the initial capital investment costs of desalination plants typically exceeds well over a billion dollars, with annual maintenance costs of $30-40 million. Desalination is also an inadequate means of providing water to the interior areas, as they require pipelines costing millions of dollars to construct and operate. Additionally, desalination systems are vulnerable to sea level rises and natural disasters such as hurricanes (e.g. the Gulf) and earthquakes (e.g. California). This is an important limitation as there have been the most significant rises in sea levels and tectonic activity in some of the most water stressed areas of the globe.
In the atmosphere, there is a natural abundance of water vapor being stored in the troposphere. Clouds of course are the most visible manifestation of atmospheric water vapor, however, even in clear air conditions there is an enormous quantity of water vapor in the air. About 0.001 percent of the Earth's total water volume is stored in the air, which upon calculation yields 1,385,000,000 km3 of water in a constant evaporation-condensation-precipitation cycle. More specifically, the troposphere contains 37,500 trillion gallons of water—a 10 year global supply. Fortunately, the water in the troposphere is replenished at a rate of 3,125 trillion gallons per day, which leads to the belief that this source of water can be a highly sustainable and renewable source of fresh water. Due to the natural cycle of evaporation, this water is already in a relatively pure state as compared to fresh surface water.
There have been attempts in the past to recover water from ambient air but these systems have had significant limitations. For example, it is well known that cooling air at or below its dew point causes condensation of water vapor from the air, resulting in a decrease in the absolute humidity of the air. Existing air-to-water generation systems which utilize this process are limited in that the quantity of water than can be produced, and are heavily hindered by the enormous energy requirements—as the process incurs heavy energetic costs to reach the dew point. Since the humidity and temperature of ambient air varies from region to region, the quantity of water that can be produced using with this technique is seldom adequate or consistent. Despite the attempts to introduce sophisticated pinch analysis techniques into this process, the thermodynamic path still remains far too energy intensive to be considered as an alternative source of water.
Another technique that has been attempted is the use of solid adsorbents such as disclosed in U.S. Pat. No. 4,365,979. These techniques pass air through a column packed with a solid adsorbent to adsorb water from the air. The water saturated adsorbent is then heated—statically or with dry hot gas—to effectively evaporate the water, and in doing so generating a saturated water vapor stream which is subsequently condensed to its liquid form. Unfortunately, this type of system continues to be subject to variation in the humidity of the ambient air, energy intensive, and difficult to keep in continuous operation. In addition, the energetic pathway relies upon two costly phase changes, evaporation and condensation, causing excessive energy consumption.
Another technique that has been attempted to produce water from ambient air is disclosed in PCT/US2005/030529 (“the Sher process”). As best can be determined, the Sher process entails removing water from the ambient air by exposing a first ambient air stream to a desiccant to increase the water content of the desiccant. This desiccant is then exposed to a second air stream to facilitate the evaporation of the water that had been absorbed by the desiccant mating a water saturated air stream. The saturated air stream is subsequently exposed to a heat exchanger to facilitate condensation. To facilitate the Sher process, the first airstream is cooled by the initial desiccant (which has been cooled below the ambient airflow) to induce a favorable temperature gradient required for adequate mass transfer into the desiccant. The second air stream which is passed over the desiccant, is heated along with the desiccant to facilitate water evaporation from the desiccant in order to generate a saturated airstream. To harvest the water from the saturated air stream in the Sher process, the stream subjected to cooling to induce a phase change through condensation. To facilitate the required heat transfer required for condensation and evaporation, a system of heat exchangers and refrigerants are utilized to support the process. The harvested water in question is then purified to potable standards.
The Sher process for harvesting atmospheric water is not dissimilar in to a desiccant wheel; whereby, the thermodynamic pathways are evidently similar. However, the Sher process utilizes well known prior art desiccant materials (both solid and liquid) capable of adsorbing the water from the ambient. There are a number of inherent deficiencies in the Sher process, including the requirement for two different ambient air streams, throughputs which are substantially limited by two separate air/desiccant contacting methods, substantial heating and cooling requirements, unfavorable mass transport and thermodynamic conditions, and higher temperature operation requirements.
As can be seen, there is a need for a system and a method of producing water from air that is versatile over various ambient air conditions, that is not as energy or capital intensive as prior art systems, and that can be in continuous operation without significant maintenance problems.